The inflammatory response serves the purpose of eliminating harmful agents from the body. There is a wide range of pathogenic insults that can initiate an inflammatory response including infection, allergens, autoimmune stimuli, immune response to transplanted tissue, noxious chemicals, and toxins, ischemia/reperfusion, hypoxia, mechanical and thermal trauma. Inflammation normally is a very localized action which serves in expulsion, attenuation by dilution, and isolation of the damaging agent and injured tissue. The body's response becomes an agent of disease when it results in inappropriate injury to host tissues in the process of eliminating the targeted agent, or responding to a traumatic insult.
The release of inflammatory cytokines such as tumor necrosis factor-alpha (TNFα) by leukocytes is a means by which the immune system combats pathogenic invasions, including infections. TNFα stimulates the expression and activation of adherence factors on leukocytes and endothelial cells, primes neutrophils for an enhanced inflammatory response to secondary stimuli and enhances adherent neutrophil oxidative activity (H. S. Sharma et al., Med. of Inflamm., 6, 175 (1987)). In addition, macrophages/dendritic cells act as accessory cells processing antigen for presentation to lymphocytes. The lymphocytes, in turn, become stimulated to act as pro-inflammatory cytotoxic cells.
Generally, cytokines stimulate neutrophils to enhance oxidative (e.g., superoxide and secondary products) and non-oxidative (e.g., myeloperoxidase and other enzymes) inflammatory activity. Inappropriate and over-release of cytokines can produce counterproductive exaggerated pathogenic effects through the release of tissue-damaging oxidative and non-oxidative products (K. G. Tracey et al., J. Exp. Med., 167, 1211 (1988); and D. N. Männel et al., Rev. Infect. Dis., 9 (suppl. 5), S602-S606 (1987)). For example, TNFα can induce neutrophils to adhere to the blood vessel wall and then to migrate through the vessel to the site of injury and release their oxidative and non-oxidative inflammatory products.
Although monocytes collect slowly at inflammatory foci, given favorable conditions, the monocytes develop into long-term resident accessory cells and macrophages. Upon stimulation with an inflammation trigger, monocytes/macrophages also produce and secrete an array of cytokines (including TNFα), complement, lipids, reactive oxygen species, proteases and growth factors that remodel tissue and regulate surrounding tissue functions.
Human rheumatoid arthritis (RA) is the most common form of inflammatory arthritis and is a chronic disorder of unknown origin with variable courses of disease. The majority of patients with RA have a progressive course which leads to destruction of joint tissue, instability of joints, loss of function and mobility, and increased mortality.
The dysregulated immune response in RA is driven by type 1 helper T cell (Th1) cytokines, including interleukin-12 (IL-12), interferon-γ (IFN-γ), and TNFα (See, M. Feldmann et al., Curr. Dir. Autoimmun., 3, 188-199 (2001); H. Schulze-Koops et al., Best. Pract. Res. Clin. Rheumatol., 15, 677-691 (2001); M. Feldmann et al., Rheumatology (Oxford), 38, 3-7 (1999); and Y. Morita et al., Arthritis Rheum., 41, 306-314 (1998)). Cytokines and chemokines in the synovial tissues trigger the adhesion, recruitment, and infiltration of inflammatory cells and the release of other inflammatory mediators and reactive oxygen species, leading to macroscopic tissue damage and the clinical symptoms of RA (Mechanisms and Models in Rheumatoid Arthritis. 1 ed. San Diego, Calif.: Academic Press (1995)). Recent studies have shown that TNFα and IL-1β are major proinflammatory cytokines in this inflammatory disorder, and are currently targets for therapeutic intervention (M. Feldmann et al., Curr. Dir. Autoimmun., 3, 188-199 (2001); M. Feldmann et al., Transplant Proc., 2001; 33, 2085-2086 (2001); M. Feldmann, Nat. Rev. Immunol., 2, 364-371 (2002); and R. Maini et al., Lancet, 348, 824-825 (1996)).
Standard RA therapy includes the use of immunomodulators, biologic agents, and corticosteroids, all of which have limited efficacy and carry significant risk of toxicity. Methotrexate is the gold standard treatment for RA.
Steroids act through multiple pathways, and besides their immunosuppressive effect, have multiple side effects including osteoporosis, worsening diabetes, and adrenal suppression. Traditional non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, aspirin and indomethacin non-selectively inhibit both COX-1 and -2. The more recently described COX-2 inhibitors (rofecoxib/Vioxx, Celecoxib/Celebrx and valdecoxib/Bextra) were developed to decrease gastric toxicity by avoiding COX-1 inhibition. These agent impart their anti-inflammatory effects through action on the COX-2 enzyme, and provide relief of pain and some mild anti-inflammatory effect, but do not stop progression of disease. The COX-2 inhibitors also avoid the platelet-inhibiting effect of traditional NSAIDS, which may play a role in the recent reports of increased cardiovascular risk with COX-2 inhibitors.
The latest strategies for the treatment of RA focus on specifically targeting the dysregulated cytokines. TNF inhibitors (Adalimumab, Etanercept, Infliximab) and the IL-1 receptor antagonist Anakinra block two of the prime mediators of inflammation, but predispose patients to bacterial and granulomatous infections. Even these sophisticated biological response modifiers have met with varying success, and often require multi-drug combination therapy to control disease. The current paradigm for RA management is to treat aggressively and early in the disease because joint swelling can rapidly proceed to erosion (bony destruction) and loss of cartilage and joint space (J. O'Dell, N. Engl. J. Med., 350, 2591-2602 (2004)). Once these bony changes occur they are essentially irreversible, and can be painful, debilitating, and may leave the patient with surgery (e.g., joint replacement) as the only option. Clearly there is a need for new therapies with improved safety as well as the identification of new targets for suppression of inflammation and immunosuppression that can be used in conjunction with other agents.
It is well known that adenosine and some analogs of adenosine that nonselectively activate adenosine receptor subtypes decrease neutrophil production of inflammatory oxidative products (B. N. Cronstein et al., Ann. N.Y. Acad. Sci., 451, 291 (1985); P. A. Roberts et al., Biochem. J., 227, 669 (1985); D. J. Schrier et al., J. Immunol., 137, 3284 (1986); B. N. Cronstein et al., Clinical Immunol. and Immunopath., 42, 76 (1987); M. A. Iannone et al., in Topics and Perspective in Adenosine Research, E. Gerlach et al., eds., Springer-Verlag, Berlin, p. 286 (1987); S. T. McGarrity et al., J. Leukocyte Biol., 44, 411421 (1988); J. De La Harpe et al., J. Immunol., 143, 596 (1989); S. T. McGarrity et al., J. Immunol., 142, 1986 (1989); and C. P. Nielson et al., Br. J. Pharmacol., 97, 882 (1989)). For example, adenosine has been shown to inhibit superoxide release from neutrophils stimulated by chemoattractants such as the synthetic mimic of bacterial peptides, f-met-leu-phe (fMLP), and the complement component C5a (B. N. Cronstein et al., J. Immunol., 135, 1366 (1985)). Adenosine can decrease the greatly enhanced oxidative burst of PMN (neutrophil) first primed with TNF-α and then stimulated by a second stimulus such as f-met-leu-phe (G. W. Sullivan et al., Clin. Res., 41, 172A (1993)). Additionally, it has been reported that adenosine can decrease the rate of HIV replication in a T-cell line (S. Sipka et al., Acta. Biochim. Biopys. Hung., 23, 75 (1988)). However, there is no evidence that in vivo adenosine has anti-inflammatory activity (G. S. Firestein et al., Clin. Res., 41, 170A (1993); and B. N. Cronstein et al., Clin. Res., 41, 244A (1993)).
It has been suggested that there is more than one subtype of adenosine receptor on neutrophils that can have opposite effects on superoxide release (B. N. Cronstein et al., J. Clin. Invest. 85, 1150 (1990)). The existence of A2A receptor on neutrophils was originally demonstrated by Van Calker et al. (D. Van Calker et al., Eur. J. Pharmacology, 206, 285 (1991)).
There is one report of the combination of relatively nonspecific adenosine analogs, R-phenylisopropyladenosine (R-PIA) and 2-chloroadenosine (Cl-Ado) with a phosphodiesterase (PDE) inhibitor resulting in a lowering of neutrophil oxidative activity (M. A. Iannone et al., Topics and Perspectives in Adenosine Research, E. Garlach et al., eds., Springer-Verlag, Berlin, pp. 286-298 (1987)). However, R-PIA and Cl-Ado analogs are actually more potent activators of A1 adenosine receptors than of A2A adenosine receptors and, thus, are likely to cause side effects due to activation of A1 receptors on cardiac muscle and other tissues causing effects such as “heart block.”
There remains a need for compounds and methods for treating an inflammatory response caused by autoimmune stimulation, particularly caused by arthritis.